This invention relates generally to an apparatus and method for acoustically testing wood properties and suitable for live, standing trees.
Currently, it is difficult to nondestructively assess wood quality in standing trees. In order to evaluate wood quality, a forester must take samples of the wood and send them to a laboratory for analysis, or fell the tree first and make a visual inspection of the external characteristics of the logs cut from the tree. Often a complete assessment of wood quality cannot be made until the timber is in the milling process. By this time, considerable resources have been spent locating and transporting the timber. Discovering that the timber is not of usable quality is a waste of these limited resources. Furthermore, otherwise useful trees are unnecessarily removed from the forest.
The measurement of sound speed through wood is a well-known method for evaluating logs and timber. Sound speed is generally related to the modulus of elasticity of the wood and may indicate additional properties related to wood strength and quality. Typically in such techniques, a resonance-based approach may be used or an acoustic signal is introduced into one end of a log or at the end or surface of exposed timber, and its time of transit to the other end or separate point is recorded. Sound speed is derived simply by dividing the transit time by the length of the sound path through the log or timber.
PCT patent application WO 02/29398 describes a variation on this technique for use with standing trees. Following the method of this patent application, shallow spikes held in separation by a rigid bar are used to introduce sound waves into the cambium of the tree at a first location and extract the sound waves at a second location a predetermined distance away from the first location. The time between a tapping of the first spike and a receipt of the sound wave at the second spike displaced by the known length of the rigid bar provides a measure of sound speed that may be used to deduce modulus of elasticity. One drawback to this system is possible acoustic contamination from sound passing not through the wood, but directly between the probes through the connecting bar.
New Zealand patent application No. 533153, filed May 26, 2004 by inventor Chin-Lin Huang, entitled: “System and Method for Measuring Stiffness in Standing Trees” and claiming priority from U.S. patent application Ser. No. 10/600,933 filed Jun. 20, 2003, describes a similar system, but in which the probes are separate, to be freely located on the sides of the tree held by spikes engaging the cambium wood. Each probe communicates by means of electrical cables to a measuring unit. Because the probes are connected only by flexible cables to the measuring unit, the sound detected by the receiving probe is not contaminated with sound through a connecting bar.
The use of two separate probes, unconnected by a bar, reduces the weight and bulk of the system, simplifying its use by a single operator in the field. Yet because the probes are no longer held in rigid separation, their separation distance, which must be known for accurate sound speed measurement, will vary. This variation can be accommodated by the additional step of having the operator measure the separation between the probes, for example, using graduations on the connecting cables as a ruler as proposed in the 533153 patent, but this added measurement introduces possible operator error and decreases the convenience and speed with which the system may be used.
When separate probes are used, they can be misaligned when installed on the tree, significantly decreasing the strength of the received acoustic signal, and hence accuracy of the measurement. The electrical connections between the probes and measuring unit, no longer routed along the separating bar, may become tangled and damaged.